A linear charger charges a battery with current, and therefore, if there is no battery, the output of the linear charger will quickly raise to a higher voltage due to the output current of the linear charger, thereby causing accumulation of heat and charges on the output terminal of the linear charger, which may damage the circuit of the linear charger. In state-of-art, there are two methods to protect a charger system, of which one is adding protection circuit to the system and the other is detecting the battery. In the second method, if there is no battery, the charger will be off or switched to another operation mode, and a no-battery signal could be sent to the system for further applications. For the second method, U.S. Pat. No. 5,291,118 to Kojima detects the no-battery condition with thermistor, U.S. Pat. No. 5,932,989 to Thandiwe et al. detects the present battery with EEPROM, and both of these arts rely on external component for the detection of battery connection. Alternatively, U.S. Pat. No. 6,340,876 to Saint-Piere detects the battery removal by the charger circuit itself, which monitors the charger positive and negative terminals to detect violate variation of charge voltage and charge current, to annunciate the current state of the charger.